foil

Flat white paint reflects better.

 

Use the normal side (not-shiny).

 

i thot that the shiny side wood be better

 

wouldnt the shiny side do better or would the light not be focused as much when its reflected

 

I believe the shinier side would be more reflective. Foil isn't the best idea, though, as it can create hotspots. Mylar or white paint are some alternate options.

~Lycanthropy

 

The shiney side will definitely cause hot-spots cos it does not diffuse light like the dull side of the foil which is better!
Both sides have about the same reflectivity values but diffusion will reflect the light more evenly, better for your plants!
White Paint is the better alternative though!
Higher reflectivity value than foil, it wont cause hot spots and it diffuses light brilliantly!
Hope that helps!
CWL